The invention relates generally to optical switching arrangements and more particularly to arrangements of thermally actuated switching units for selectively manipulating optical signals from input and add ports to output and drop ports.
While signals within telecommunications and data communications networks have been traditionally exchanged by transmitting electrical signals via electrically conductive lines, an alternative mode of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulations of laser-produced light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals is sometimes satisfied by converting the optical signals to electrical signals at the inputs of a switching network, and then reconverting the electrical signals to optical signals at the outputs of the switching network.
Recently, reliable optical switching systems have been developed. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from any one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. An isolated switching unit 10 is shown in FIG. 1. The switching unit includes planar waveguides that are formed by layers on a substrate. The waveguide layers include a lower cladding layer 14, an optical core 16, and an upper cladding layer, not shown. The optical core is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers are formed of a material having a refractive index that is substantially different than the refractive index of the core material, so that optical signals are guided along the core.
The layer of core material 16 is patterned into waveguide segments that define a first input waveguide 20 and a first output waveguide 26 of a first optical path and define a second input waveguide 24 with a second output waveguide 22 of a second optical path. The upper cladding layer is then deposited over the patterned core material. A gap is formed by etching a trench 28 through the core material, the upper cladding layer, and at least a portion of the lower cladding layer 14. The first input waveguide 20 and the second output waveguide 22 intersect a sidewall of the trench 28 at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the crosspoint 30 of the waveguides is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide 20 to the output waveguide 22, unless an index-matching fluid resides within the crosspoint 30 between the aligned input and output waveguides 20 and 26. The trench 28 is positioned with respect to the four waveguides such that one sidewall of the trench passes through or is slightly offset from the intersection of the axes of the waveguides.
The above-identified patent to Fouquet et al. describes a number of alternative approaches to switching the optical switching unit 10 between a transmissive state and a reflective state. One approach is illustrated in FIG. 1. The switching unit 10 includes a microheater 38 that controls formation of a bubble within the fluid-containing trench. While not shown in the embodiment of FIG. 1, the waveguides of a switching matrix are typically formed on a waveguide substrate and the heaters and heater control circuitry are integrated onto a heater substrate that is bonded to the waveguide substrate. The fluid within the trench has a refractive index that is close to the refractive index of the core material 16 of the four waveguides 20-26. Fluid fill-holes 34 and 36 may be used to provide a steady supply of fluid, but this is not critical. In the operation of the switching unit, the heater 38 is brought to a temperature sufficiently high to form a bubble. Once formed, the bubble can be maintained in position by maintaining power to the heater. In FIG. 1, the bubble is positioned at the crosspoint 30 of the four waveguides. Consequently, an input signal along the waveguide 20 will encounter a refractive index mismatch upon reaching the sidewall of the trench 28. This places the switching unit in a reflective state, causing the optical signal along the waveguide 20 to be redirected to the second output waveguide 22. However, even in the reflective state, the second input waveguide 24 is not in communication with the first output waveguide 26.
If the heater 38 at crosspoint 30 is deactivated, the bubble will quickly condense and disappear. This allows index-matching fluid to fill the crosspoint 30 of the waveguides 20-26. The switching unit 10 is then in the transmissive state, since input signals will not encounter a significant change in refractive index at the interfaces of the input waveguides 20 and 24 with the trench 28. In the transmissive state, the optical signals along the first input waveguide 20 will propagate through the trench to the first output waveguide 26, while optical signals that are introduced via the second input waveguide 24 will propagate through the trench to the second output waveguide 22.
Matrices of the switching elements 10 may be used to form complex switching arrangements., A switching matrix may have any number of input ports (N) and any number of output ports (M), With each port being connected to an optical fiber. The fluid-controlled switching units allow the arrangement to be a strictly xe2x80x9cnon-blockingxe2x80x9d matrix, since any free input fiber can be optically coupled to any free output fiber without rearrangement of existing connections.
Another type of switching matrix is an add/drop multiplexer that includes add ports and drop ports in addition to the input and output ports. Such multiplexers are utilized in telecommunications applications in which signals are passed through a series of nodes, with each node being able to introduce additional signals and being able to extract those signals that identify that node as a target. For example, each node may be a switching facility of a long distance carrier that supports calls to and from a number of cities. Calls that originate in a city are introduced using add ports within the switching facility of that city. On the other hand, data and voice information for calls directed to a telephone supported by that switching facility are extracted via drop ports. A known switching arrangement 42 that can be used as a rearrangeable add/drop switch is shown in FIG. 2. The arrangement includes a 4xc3x974 matrix of optical switching units for selectively coupling any one of four input ports 44, 46, 48 and 50 to any one of four output ports 52, 54, 56 and 58. In FIG. 2, each of the switching units that is in a reflective state is shown as having a bubble at the area of the intersection of input and output waveguides to that switching unit. Thus, switching units 60, 62, 64 and 66 are in reflective states. The remaining twelve switching units are in the transmissive state, since there are no bubbles present at the intersections of the input and output waveguides to those switching units.
Optical fibers are connected to each of the input ports 44-50 and each of the output ports 52-58. An optical signal that is introduced at the input port 44 will be reflected at the switching unit 62 and will be output via the output port 54. Similarly, an optical signal from the input port 46 will reflect at the switching unit 64 for output at the port 56. An optical signal from input port 48 reflects at the switching unit 66 for output via the port 58. Finally, an optical signal on port 50 is reflected to output port 52 by the switching unit 60. By selectively manipulating the bubbles within the various trenches, any one of the input ports can be connected to any one of the output ports.
The arrangement 42 includes four add ports 68, 70, 72 and 74. Each add port is uniquely associated with one of the output ports 52-58, since an optical signal that is introduced at one of the add ports can be directed only to its aligned output port. Thus, an optical signal on add port 68 can be directed to the output port 52 by changing the switching unit 60 to the transmissive state. This change to the transmissive state places the input port 50 in optical communication with a drop port 76. The drop port 76 is uniquely associated with the input port 50, since the drop port cannot be optically coupled to any other input or add port. Similarly, each one of three other drop ports 78, 80 and 82 is uniquely associated with the input port 44, 46 and 48, respectively, with which the drop port is linearly aligned.
A concern with the optical arrangement 42 of FIG. 2 is that it allows a limited flexibility with regard to introducing and extracting signals. What is needed is an optical switching arrangement that has a high degree of flexibility with respect to channeling optical signals from input ports to drop ports and from add ports to output ports. Another concern with the prior art arrangement is that the differences in the path lengths and the number of switching units that must be traversed to link a particular input port to a particular output port lead to non-uniform signal strength losses. Therefore, what is also needed is an arrangement that promotes uniformity of signal losses.
Both an increased flexibility in manipulating optical signals within a waveguide arrangement having thermally actuated switching units and a greater uniformity in the loss characteristics along transmission paths within the arrangement are achieved by forming at least two sets of fluid-containing trenches, with the difference between the sets being the direction of the offset of trenches relative to crosspoints of waveguides. That is, the spatial relationship of first trenches (e.g., left-aligned trenches) relative to waveguide crosspoints with which they are associated is the mirror image of the spatial relationship of second trenches (e.g., right-aligned trenches) to the waveguide crosspoints with which they are associated. As a result, the reflection characteristics of the first trenches are a mirror image of the reflection characteristics of the second trenches.
In the preferred embodiment, the optical arrangement is an add/drop switch having an array of generally parallel first optical paths with input ports at first ends and output ports at opposite ends. Each optical path is formed of generally aligned waveguides that intersect the fluid-containing trenches. The first optical paths are intersected at the first trenches by drop paths. Each drop path has a drop port at one end and is formed of generally aligned waveguides. The first optical paths are intersected at the second trenches by add paths. The add paths are formed by waveguides and include add ports. By manipulating the fluid within the first trenches, any input port can be optically coupled to any of the drop ports. Similarly, by manipulating fluid within the second trenches, any of the add ports can be optically coupled to any of the output ports.
Each trench defines a switching unit that is responsive to the manipulation of index-matching fluid to change between a reflective state and a transmissive state. Typically, the fluid is a liquid having a refractive index that closely matches the refractive index of the optical core material of the waveguides. Consequently, when fluid resides at the interface of a trench with a waveguide, an optical signal propagating through the waveguide will enter the trench and will propagate through the trench to the waveguide that is at the opposite sidewall of the trench. On the other hand, when there is an absence of liquid at the waveguide-to-trench interface, the trench is in a reflective state and any optical signal that reaches the interface will be reflected. In the preferred embodiment in which the arrangement is an add/drop switch, the reflection by the first trenches is from an input port to a drop port, while the reflection at a second trench is from an add port to an output port.
In one embodiment, the number (M) of input ports is equal to the number of output ports and is twice the number (M/2) of add ports and twice the number of drop ports. Each drop path then intersects M first trenches, and each add path intersects M second trenches. However, other embodiments are contemplated.
An advantage of the invention is that by providing different sets of trenches having mirror-image reflection characteristics on a single substrate, the switching arrangement enables a flexible selection of signal manipulations. In the add/drop switch embodiment, xe2x80x9caddxe2x80x9d signals can be switched to any one of the available output ports without rearrangement of any existing connections. Moreover, any input signal can be switched to any one of the available drop ports without rearrangement of existing connections. Another advantage of the preferred embodiment is that all of the paths from the input ports to the associated output ports are identical with respect to path length and the number of trenches that must be traversed. As a result, all of the input signals that propagate from the input ports to the output ports experience a uniform insertion loss.